Really rough, but works well enough. Almost of an exact port of https://github.com/rodchile/UITextCurrencyField.

MonoTouch.Dialog Currency Element

	// Currency element, port of: https://github.com/rodchile/UITextCurrencyField
	public class CurrencyElement : EntryElement
	{
		public CurrencyElement (string caption, string placeholder, string value)
			: base(caption, placeholder, value)
		{

		}

		private static string CleanString(string target)
		{
			string returnValue = string.Empty;
			foreach (char c in target) {
				if (char.IsDigit (c) || char.IsWhiteSpace (c)) {
					returnValue += c;
				}
			}
			return returnValue;
		}

		protected override UITextField CreateTextField (RectangleF frame)
		{
			NSLocale current = NSLocale.CurrentLocale;
			string currencySymbol = current.CurrencySymbol;
			string groupingSeperator = current.GroupingSeparator;

			var formatter = CurrencyFormatter.Currency;
			var basic = CurrencyFormatter.Basic;

			UITextField field = base.CreateTextField (frame);

			if (!string.IsNullOrEmpty (Value)) {
				field.Text = Value;
			} else {
				string initialValue = formatter.StringFromNumber (0);
				field.Text = initialValue;
			}

			field.ShouldChangeCharacters += delegate (UITextField textField, NSRange range, string replacementString) {
				bool result = false;
				if (string.IsNullOrEmpty(replacementString)) {
					result = true;
				} else {
					string clean = CleanString(replacementString);
					if (!string.IsNullOrEmpty(clean)) {
						result = true;
					}
				}

				if (result) {
					string mstring = textField.Text;

					if (!string.IsNullOrEmpty(replacementString)) {
						mstring = mstring.Insert(range.Location, replacementString);
					} else {
						mstring = mstring.Remove(range.Location, range.Length);
					}

					string cleanString = mstring.Replace(groupingSeperator, "")
						.Replace(currencySymbol, "");

					if (formatter.MaximumFractionDigits > 0) {
						string mutableCleanString = cleanString;

						if (!string.IsNullOrEmpty(replacementString)) {
							int index = mutableCleanString.IndexOf(".");
							mutableCleanString = mutableCleanString.Replace(".", "");
							mutableCleanString = mutableCleanString.Insert(index + 1, ".");
							cleanString = mutableCleanString;
						} else {
							mutableCleanString = mutableCleanString.Insert(0, "0");
							int index = mutableCleanString.IndexOf(".");
							mutableCleanString = mutableCleanString.Replace(".", "");
							mutableCleanString = mutableCleanString.Insert(index - 1, ".");
							cleanString = mutableCleanString;
						}
					}

					NSNumber number = basic.NumberFromString(cleanString);
					string numberString = formatter.StringFromNumber(number);
					textField.Text = numberString;

					if (EditingChanged != null) {
						EditingChanged(this, EventArgs.Empty);
					}
				}

				return false;
			};
			return field;
		}
	}

Did you have a CurrencyFormatter file or class?, or i dont know what .net namespace is.

Oh, i can see what is:
NSNumberFormatter* currencyFormatter = [[[NSNumberFormatter alloc] init] autorelease];
[currencyFormatter setFormatterBehavior:NSNumberFormatterBehavior10_4];
[currencyFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];
[currencyFormatter setMaximumFractionDigits:0];
[currencyFormatter setLocale:[NSLocale currentLocale]];